Inner focus lens

ABSTRACT

An inner focus lens includes sequentially from an object side, a first lens group having a positive refractive power; a second lens group having a negative refractive power; and a third lens group having a negative refractive power. The second lens group is moved along the optical axis, whereby focusing from a focus state for an object at infinity to a focus state for a minimum object distance is performed. The inner focus lens satisfies a conditional expression (1) −29.0≦f3/f≦− 5.4 , where f3 is a focal length of the third lens group at the focus state for an object at infinity and f is a focal length of the optical system overall at the focus state for an object at infinity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a compact inner focus lens having highimaging performance.

2. Description of the Related Art

Conventionally, a long flange focal length relative to the focal lengthhas to be established and thus, many lenses for single-lens reflexcameras adopt a configuration that includes a positive lens group towardthe rear of the optical system to easily establish back focus.Nonetheless, in recent years, camera bodies have decreased in size andconsequent to the spread of digital cameras, instances where a longflange focal length is not necessary are increasing.

Further, since video filming is also possible by a digital camera,high-speed autofocus processing for video filming is desirable. Aportion of a lens group (focusing group) is moved rapidly along theoptical axis (wobble) to achieve transitions: a non-focusedstate→focused state→non-focused state. A signal component of a specificfrequency band of a partial image area is detected from the outputsignal of the image sensor; an optimal position of the focusing groupachieving a focused state is determined; and the focusing group is movedto the optimal position. In particular, with video filming, this seriesof operations has to be rapidly continued, repeatedly. Further, in theexecution of wobble, rapid driving of the focusing group has to bepossible and the focusing group is demanded to have the smallestdiameter possible and to be light-weight.

To address such demands, an inner focus lens that can also sufficientlycope with video filming has been proposed (for example, refer toJapanese Patent Application Laid-Open Publication No. 2013-97212).

The inner focus lens disclosed in Japanese Patent Application Laid-OpenPublication No. 2013-97212 has a medium telephoto focal length by a 35mm film camera conversion and a small, light-weight focusing groupinternally, thereby enabling favorable wobble to be executed.

On the other hand, conventionally, at the image sensor that opticallyreceives and converts an optical image into an electronic image signal,there are limitations for efficiently taking in incident light by theon-chip microlens, etc. and on the lens side, the exit pupil is made tobe greater than a certain diameter and assured telecentricity of theluminous flux incident to the image sensor is desirable.

Nonetheless, with recent image sensors, improved aperture ratios andadvances in the design freedom of on-chip microlenses have reduced theexit pupil limitations demanded on the image lens side. Furthermore,with recent software and camera system advances and improvements, evenwhen distortion is significant to an extent that conventionally, thedistortion would be conspicuous, correction by image processing hasbecome possible.

Therefore, in conventional image lenses, although a positive lenselement is disposed farthest on the image side of the optical system andtelecentricity is assured, in recent years, this is no longer necessaryand even when a negative lens element is disposed farthest on the imageside of the optical system and there is oblique incidence of theluminous flux on the image sensor, vignetting (shading) consequent tomismatching of the on-chip microlens and pupil, etc. has becomeinconspicuous. Further, since a negative lens element can now bedisposed farthest on the image side of an optical system, reductions inthe diameter of optical systems can be expected.

In contrast, with the inner focus lens disclosed in Japanese PatentApplication Laid-Open Publication No. 2013-97212, since a positive lenselement is disposed farthest on the image side of the optical system,which has a shorter overall length, the diameter of the third lens group(lens farthest on the image side) cannot be sufficiently reduced.Therefore, provision of an inner focus lens for cameras having a smallerdimension along the direction of the diameter of the optical system isdifficult, including for mirrorless interchangeable-lens cameras thathave come into wide use. Furthermore, when focusing is performed, tosuppress aberration variations consequent to wobble and the effects ofmagnification, disposal of a negative element farthest on the image sideof the optical system is desirable.

The inner focus lens disclosed in Japanese Patent Application Laid-OpenPublication No. 2013-97212 is not aimed for wide angle views andtherefore, the correction of field curvature and distortion as well asassuring the amount of light at lens edges necessary for wide angleviews are points that have not been considered.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the aboveproblems in the conventional technologies.

An inner focus lens includes sequentially from an object side, a firstlens group having a positive refractive power; a second lens grouphaving a negative refractive power; and a third lens group having anegative refractive power. The second lens group is moved along theoptical axis, whereby focusing from a focus state for an object atinfinity to a focus state for a minimum object distance is performed.The inner focus lens satisfies a conditional expression (1)−29.0≦f3/f≦−5.4, where f3 is a focal length of the third lens group atthe focus state for an object at infinity and f is a focal length of theoptical system overall at the focus state for an object at infinity.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting, along the optical axis, a configurationof an inner focus lens according to a first embodiment;

FIG. 2 is a diagram of various types of aberration occurring in theinner focus lens according to the first embodiment;

FIG. 3 is a diagram depicting, along the optical axis, a configurationof the inner focus lens according to a second embodiment;

FIG. 4 is a diagram of various types of aberration occurring in theinner focus lens according to the second embodiment;

FIG. 5 is a diagram depicting, along the optical axis, a configurationof the inner focus lens according to a third embodiment; and

FIG. 6 is a diagram of various types of aberration occurring in theinner focus lens according to the third embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of an inner focus lens according to the presentinvention will be described in detail.

The inner focus lens according to the present invention is configured bya first lens group having a positive refractive power, a second lensgroup having a negative refractive power, and a third lens group havinga negative refractive power, sequentially arranged from an object side.

In the inner focus lens according to the present invention, the secondlens group is moved along the optical axis whereby focusing from a focusstate for an object at infinity to a focus state for the minimum objectdistance is performed. In this manner, by moving the second lens groupto perform focusing, protection against dust and sound-proofingperformance are enhanced without changes in the overall length of theoptical system.

Further, by disposing farthest on the object side, the first lens grouphaving a positive refractive power, the diameter of the luminous fluxguided to the subsequent second lens group can be reduced. Therefore,the diameter of the second lens group, which is the focusing group, isreduced, enabling a reduction in the weight of second lens group to befacilitated. As a result, high-speed, silent focusing becomes possible,which is effective for video filming. Further, since the diameter of thesecond lens group can be reduced, this is advantageous for reducing thediameter of the optical system.

Moreover, by disposing farthest on the image side, the third lens grouphaving a negative refractive power, telecentricity can be increased andback focus can be reduced, enabling size reductions of optical system tobe facilitated.

One object of the present invention is to provide an inner focus lensthat is also optimal for a compact camera capable of video filming,i.e., a high imaging performance inner focus lens for which reduction ofthe overall length and diameter is achieved. Another object is toprovide a high imaging performance inner focus lens having focal lengthsthat accommodate wide angles to standard angles of view. Thus, toachieve such objects, in addition to the characteristics above, variousconditions are set such as those indicated below.

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, f3 isthe focal length of the third lens group at a focus state for an objectat infinity; and f is the focal length of the optical system overall ata focus state for an object at infinity.

−29.0≦f3/f≦−5.4  (1)

Conditional expression (1) prescribes a ratio of the focal lengths ofthe third lens group and of the optical system, at a focus state for anobject at infinity. Satisfaction of conditional expression (1) enablesreductions in the overall length and diameter of the optical system aswell as enables the refractive power of the third lens group to beproper and without deterioration of imaging performance.

Below the lower limit of conditional expression (1), the refractivepower of the third lens group becomes weak, whereby the back focusincreases, making reductions in the size of the optical systemdifficult. On the other hand, above the upper limit of conditionalexpression (1), the refractive power of the third lens group becomesstrong. In this case, the F number in the optical system overall tendsto increase, making a bright optical system impossible to obtain. Torealize a bright optical system in this state, the aperture stop has tobe opened greatly. Nonetheless, if the aperture stop is opened greatly,the occurrence of various types of aberration becomes conspicuous andtherefore, to realize an optical system having favorable imagingperformance, the number of lenses for correcting aberration has to beincreased. In particular, the number of lenses forming the first lensgroup has to be increased. If the number of lenses forming the opticalsystem is high, reductions in the size and weight of the optical systembecome difficult and therefore, are not desirable.

By satisfying conditional expression (1) within the following range, amore favorable effect can be expected.

−26.0≦f3/f≦−5.4  (1a)

By satisfying the range prescribed by conditional expression (1a), aninner focus lens that is compact and has better imaging performance canbe realized.

By further satisfying conditional expression (1a) within the followingrange, an even more compact, higher performance inner focus lens can berealized.

−24.0≦f3/f≦−5.4  (1b)

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, f1 isthe focal length of the first lens group at a focus state for an objectat infinity; and f is the focal length of the optical system overall ata focus state for an object at infinity.

0.18≦f1/f5≦0.99  (2)

Conditional expression (2) prescribes a ratio of the focal lengths ofthe first lens group and of the optical system overall, at a focus statefor an object at infinity. Satisfaction of conditional expression (2)enables a bright inner focus lens to be realized that has high imagingperformance at wide angles and for which the refractive power of thefirst lens group is proper and reductions of the front lens diameter andoverall length of the optical system are facilitated.

Below the lower limit of conditional expression (2), the focal length ofthe first lens group becomes short, positive spherical aberrationbecomes excessive, paraxial magnification of the subsequent lens groupbecomes large, and the rear lens diameter increases, which are relatedto increases in the size of the optical system and therefore, are notdesirable. On the other hand, above the upper limit of conditionalexpression (2), the focal length of first lens group becomes long,increasing the overall length of the optical system and making sizereductions of the optical system difficult.

By satisfying conditional expression (2) within the following range, amore favorable effect can be expected.

0.22≦f1/f≦0.90  (2a)

By satisfying the range prescribed by conditional expression (2a), aninner focus lens that is compact and has better imaging performance atwide angles can be realized.

By further satisfying conditional expression (2a) within the followingrange, a compact inner focus lens with higher performance at wide anglescan be realized.

0.30≦f1/f≦0.80  (2b)

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, βinf isthe paraxial magnification of the second lens group at a focus state foran object at infinity; and βmod is the paraxial magnification of thesecond lens group at a focus state for the minimum object distance.

0.51≦βinf/βmod≦2.07  (3)

Conditional expression (3) prescribes a ratio of the paraxial transversemagnification of the second lens group at a focus state for an object atinfinity and at a focus state for the minimum object distance.Satisfaction of conditional expression (3) enables changes inmagnification to be suppressed even when the focusing group (second lensgroup) is moved and enables angle of view variations to be suppressedduring focusing. If the range prescribed by conditional expression (3)is deviated from, angle of view variations cannot be suppressed duringfocusing. If angle of view variation occurs while the focusing group ismoving, the image looks blurred and the image quality drops.

By satisfying conditional expression (3) within the following range, amore favorable effect can be expected.

0.60≦βinf/βmod≦1.80  (3a)

By satisfying the range prescribed by conditional expression (3a), angleof view variations during focusing can be suppressed further.

By further satisfying conditional expression (3a) within the followingrange, angle of view variations during focusing can be suppressed evenfurther.

0.68≦βinf/βmod≦1.60  (3b)

By further satisfying conditional expression (3b) within the followingrange, angle of view variations during focusing can be made extremelysmall.

0.80≦βinf/βmod≦1.40  (3c)

In the inner focus lens according to the present invention, the thirdlens group includes sequentially from the object side, a front sub-lensgroup having a positive refractive power and a rear sub-lens grouphaving a negative refractive power; and an axial air gap that is thewidest in the third lens group is formed between the front sub-lensgroup and the rear sub-lens group.

By such a configuration, the lens diameter near the imaging plane isreduced and imaging performance can be improved. In other words,increased lens diameter on the image side (an issue in reducing the sizeof optical systems having a short flange focal length and for mountingon compact cameras such as mirrorless interchangeable-lens cameras) canbe suppressed by disposing the front sub-lens group having a positiverefractive power, on the object side of the third lens group. Further,by disposing the rear sub-lens group having a negative refractive poweron the image side of the front sub-lens group to form an air gap, axialaberration can be corrected by the front sub-lens group having apositive refractive power; and at the rear sub-lens group, off axisaberration, in particular, distortion, can be favorably corrected.

In the inner focus lens according to the present invention, a simplelens element having a negative refractive power is preferably disposedfarthest on the image side of the third lens group. With such aconfiguration, the diameter of the third lens group (the lens fartheston the image side) can be further reduced, which is optimal for compactcameras such as mirrorless interchangeable-lens cameras that have comeinto wide-spread use in recent years.

A simple lens element includes a single ground lens, an aspheric lens, acompound aspheric lens, and a cemented lens, where the elements are notbonded to one another to have a layer of air therebetween and forexample, a simple lens element does not include two lenses (a positivelens and a negative lens), etc.

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied, where inaddition to disposing farthest on the image side of the third lensgroup, a simple lens element having a negative refractive power, R1 isthe radius of curvature of the surface at the air interface on theobject side of the simple lens element having a negative refractivepower; and R2 is the radius of curvature of the surface at the airinterface on the image side of the simple lens element having a negativerefractive power.

(R1+R2)/(R1−R2)≦0.0  (4)

Conditional expression (4) prescribes the shape of the simple lenselement having a negative refractive power and disposed farthest on theimage side of the third lens group. By satisfying conditional expression(4), the radius of curvature of the surface on the object side of thesimple lens element becomes smaller than the radius of curvature of thesurface on the image side. As a result, favorable correction of off axiscoma becomes possible.

By satisfying conditional expression (4) within the following range, amore favorable effect can be expected.

(R1+R2)/(R1−R2)≦−1.0  (4a)

By satisfying the range prescribed by conditional expression (4a),favorable correction of off axis coma becomes possible.

By satisfying conditional expression (4a) within the following range,more effective correction of off axis coma is achieved.

−100.00≦(R1+R2)/(R1−R2)≦−1.02  (4b)

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, L1s isthe axial distance from the surface farthest on the object side of thefirst lens group to the aperture stop; L is the overall length of theoptical system (air-conversion optical path length from the apex of thelens surface farthest on the object side to the imaging plane).

0.01≦L1s/L≦0.53  (5)

Conditional expression (5) prescribes a ratio of the axial distance fromthe surface farthest on the object side of the first lens group to theaperture stop and the overall length of the optical system. Bysatisfying conditional expression (5), an optimal position of theaperture stop is determined with respect to the overall length of theoptical system, enabling reduction of the optical system diameter whilemaintaining high imaging performance.

Below the lower limit of conditional expression (5), the aperture stopis too close to the object side, the lens diameter on the image sideincreases, and at the rear group, the occurrence of off axis aberration,primarily distortion, becomes conspicuous and therefore, is notdesirable. On the other hand, above the upper limit of conditionalexpression (5), the aperture stop is to too close to the image side andwith the increase in the effective diameter of the front lens, sizereductions of the optical system become difficult.

By satisfying conditional expression (5) within the following range, amore favorable effect can be expected.

0.012≦L1s/L≦0.500  (5a)

By satisfying the range prescribed by conditional expression (5a),further reduction in the optical system diameter can be realized whilemaintaining high imaging performance.

By satisfying conditional expression (5a) within the following range,even further reduction of the optical system diameter can be realized.

0.013≦L1s/L≦0.400  (5b)

By satisfying conditional expression (5b) within the following range,yet further reduction of the optical system diameter can be realized.

0.013≦L1s/L≦0.300  (5c)

In the inner focus lens according to the present invention, the secondlens group is preferably configured by a simple lens element having anegative refractive power.

By forming the second lens group by a simple lens element having anegative refractive power, reductions in the size and weight of thefocusing group are achieved, enabling high-speed focusing, which isbeneficial for video filming. Reductions in the size and weight of thefocusing group decrease the load on the driving mechanism such as anactuator for driving the focusing group, contributing to reduced powerconsumption. The capacity of the driving mechanism can be furtherreduced.

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, f2 isthe focal length of the second lens group at a focus state for an objectat infinity; and f is the focal length of the optical system at a focusstate for an object at infinity.

−2.12≦f2/f≦−0.18  (6)

Conditional expression (6) prescribes a ratio of the focal lengths ofthe second lens group and of the optical system overall, at a focusstate for an object at infinity. By satisfying conditional expression(6), size reductions of the optical system can be realized whilemaintaining high imaging performance (particularly beneficial for fieldcurvature correction).

Below, the lower limit of conditional expression (6), the focal lengthof the second lens group becomes too long and the negative power of thesecond lens group becomes too weak. As a result, the distance that thesecond lens group has to move during focusing increases, increasing theoverall length of the optical system and making size reductions of theoptical system difficult. On the other hand, above the upper limit ofconditional expression (6), the focal length of the second lens groupbecomes too short and the negative power of the second lens groupbecomes too strong. As a result, aberration variations (particularlyfield curvature variations) accompanying the movement of the second lensgroup during focusing and variation of the angle of view becomeexcessive and are not desirable.

By satisfying conditional expression (6) within the following range, amore favorable effect can be expected.

−1.90≦f2/f≦−0.19  (6a)

By satisfying the range prescribed by conditional expression (6a), asmaller inner focus lens having excellent imaging performance can berealized.

By satisfying conditional expression (6a) within the following range, asmaller, high-performance inner focus lens can be realized.

−1.50≦f2/f≦−0.20  (6b)

In the inner focus lens according to the present invention, a lens(stabilizing group) that is configured by a lens other than the lensfarthest on the object side is moved in an orthogonal direction withrespect to the optical axis to shift the image and perform stabilizationcorrection. The inner focus lens preferably satisfies the followingconditional expression; where, βp is the transverse magnification of thelens group moved in a direction orthogonal to the optical axis; and βris the composite transverse magnification of the lenses disposed fartheron the image side than the lens group that is moved in a directionorthogonal to the optical axis.

0.15≦(1−βp)×βr≦4.50  (7)

Conditional expression (7) prescribes a shift ratio of the image, forthe moving distance of the lens group that is moved during stabilizationcorrection. By satisfying conditional expression (7), the distance thatthe stabilizing group is moved during stabilization correction issuppressed, enabling reduction of the optical system diameter andimproved stabilization correction performance. If the lens that isdisposed farthest on the image side is included in the stabilizinggroup, the value of βr in conditional expression (7) is 1.

Below the lower limit of conditional expression (7), the distance thatthe stabilizing group has to be moved in an orthogonal direction toshift the image by a given amount increases, the optical system diameterbecomes large, and size reductions of the optical system are inhibited.On the other hand, above the upper limit of conditional expression (7),even if the stabilizing group is moved minimally, the image shiftsgreatly and consequently, the stabilization correction performancedrops. In this state, if high stabilization correction performance is tobe maintained, extremely high precision is required in controlling thestabilizing group during stabilization correction. As a result, thestructure of the driving apparatus of the stabilizing group becomescomplicated, which bounces back on the manufacturing cost of the lensunit and therefore, is not desirable.

Irrespective of whether the stabilizing group is configured by multiplelenses or a single lens, the effects of stabilization correction do notchange. When the stabilizing group is configured by a single lens, thesize and weight of the stabilizing group can be reduced, which isbeneficial in reducing the overall size and weight of the opticalsystem. By reducing the size and weight of the stabilizing group, theload on the driving mechanism for driving the stabilizing group alsodecreases, contributing to reduced power consumption. Further, byemploying for the stabilizing group, an aspheric lens (a shape thatweakens the power of paraxial curvature), variation of single-sidedblurring and central coma during stabilization correction can besuppressed.

By satisfying conditional expression (7) within the following range, amore favorable effect can be expected.

0.16≦(1−βp)×βr≦4.30  (7a)

By satisfying the range prescribed by conditional expression (7a), acompact inner focus lens having better stabilization correctionperformance can be realized.

By satisfying conditional expression (7a) within the following range, acompact inner focus lens having extremely favorable stabilizationcorrection performance can be realized.

0.16≦(1−βp)×βr≦4.00  (7b)

In the inner focus lens according to the present invention, the simplelens element having a negative refractive power that is disposedfarthest on the image side of the third lens group is preferablyconfigured by a simple glass material. By forming the simple lenselement in the third lens group by a simple glass material, i.e., asimple lens, size reductions along the direction of the optical axis andalong the direction of the diameter of the simple lens element becomeeasy. Reduction in the weight of the simple lens element also becomespossible.

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, yen isthe Abbe number for e-line of the simple lens element having a negativerefractive power and disposed farthest on the image side of the thirdlens group.

30≦νen  (8)

Below the lower limit of conditional expression (8), chromaticdifference of magnification becomes over corrected, making high imagingperformance difficult to maintain and therefore, is not desirable.

In the inner focus lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, R21 isthe radius of curvature of the surface farthest on the image side of thesecond lens group and R22 is the radius of curvature of the surfacefarthest on the image side of the second lens group.

0≦(R21+R22)/(R21−R22)  (9)

Conditional expression (9) prescribes the shapes of the surfacesfarthest on the object side and on the image side of the second lensgroup. By satisfying conditional expression (9), in the second lensgroup, the radius of curvature of the surface farthest on the image sidebecomes smaller than the radius of curvature of the surface farthest onthe object side. As a result, variation of the angle of the light raysincident on the surface having a strong power becomes small, enablingvariation of the field curvature during focusing to be suppressed.

By satisfying conditional expression (9) within the following range,more favorable results can be expected.

1≦(R21+R22)/(R21−R22)  (9a)

By satisfying the range prescribed by conditional expression (9a),variation of the field curvature during focusing can be furthersuppressed.

By satisfying conditional expression (9a) within the following range,variation of the field curvature can be made extremely small.

1≦(R21+R22)/(R21−R22)≦300  (9b)

In the inner focus lens according to the present invention, disposal apositive aspheric lens in the first lens group is beneficial incorrecting spherical aberration. In particular, by forming on thepositive lens, an aspheric surface, which weakens the power of paraxialcurvature, the effectiveness of spherical aberration correction isimproved.

In the inner focus lens according to the present invention, by formingan aspheric surface on a lens forming the second lens group, correctionof field curvature becomes more effective. In particular, by forming ona lens forming the second lens group, an aspheric surface, which weakensthe power of paraxial curvature, correction of field curvature isfurther improved and the effect of suppressing field curvature variationduring focusing becomes higher.

In the inner focus lens according to the present invention, forming anaspheric surface on a lens of the third lens group is beneficial incorrecting field curvature. In particular, by forming on a lens of thethird lens group, an aspheric surface, which weakens the power ofparaxial curvature, the corrective effect on field curvature isimproved.

The lens disposed on the image side separated by air from the secondlens group is preferably a simple lens element having a positiverefractive power. Disposal of the simple lens element on the image sideof the second lens group enables magnification of the second lens groupto be increased and the distance that the second lens group is movedduring focusing to be decreased. As a result, reductions in the size ofthe optical system and high-speed focusing become possible.

As described, according to the present invention, reductions in theoverall length and diameter are achieved and an inner focus lens havinghigh imaging performance can be provided. Furthermore, an inner focuslens that has focal lengths accommodating wide angles to standard anglesof view and high imaging performance can be provided. Moreover, acompact inner focus lens having favorable stabilization correctionperformance can be provided. Thus, according to the present invention,an inner focus lens that can be mounted easily on a compact cameracapable of video filming can be provided. In particular, by satisfyingthe conditional expressions above, an inner focus lens that is optimalfor video filming and has high imaging performance with a smaller sizecan be realized.

Embodiments of the inner focus lens according to the present inventionwill be described in detail with reference to the accompanying drawings.The invention is not limited by the embodiments below.

FIG. 1 is a diagram depicting, along the optical axis, a configurationof the inner focus lens according to a first embodiment. FIG. 1 depictsa focus state for an object at infinity. The inner focus lens includessequentially from an object side nearest a non-depicted object, a firstlens group G₁₁ having a positive refractive power, a second lens groupG₁₂ having a negative refractive power, and a third lens group G₁₃having a negative refractive power.

The first lens group G₁₁ includes sequentially from the object side, alens L₁₁₁ having no refractive power, an aperture stop S determining agiven diameter, a positive lens L₁₁₂, a negative lens L₁₁₃, and apositive lens L₁₁₄. The positive lens L₁₁₂ and the negative lens L₁₁₃are cemented. Both surfaces of the positive lens L₁₁₄ are aspheric.

The second lens group G₁₂ is configured by a negative lens L₁₂₁. Bothsurfaces of the negative lens L₁₂₁ are aspheric. The second lens groupG₁₂ is moved along the optical axis from the object side to the imageside, whereby focusing from a focus state for an object at infinity to afocus state for the minimum object distance is performed.

The third lens group G₁₃ includes sequentially from the object side, apositive lens L₁₃₁ (front sub-lens group) and a negative lens L₁₃₂ (rearsub-lens group). Both surfaces of the positive lens L₁₃₁ are aspheric.An air gap is formed between the positive lens L₁₃₁ and the negativelens L₁₃₂.

In the inner focus lens according to the first embodiment, the positivelens L₁₁₄ included in the first lens group G₁₁ or the positive lens L₁₃₁included in the third lens group G₁₃ is moved in a direction orthogonalto the optical axis, whereby stabilization correction is performed.Further, stabilization correction can be performed by collectivelymoving all of the lenses of the first lens group G₁₁ excluding the lensL₁₁₁ having no refractive power in a direction orthogonal to the opticalaxis.

Here, various values related to the inner focus lens according to thefirst embodiment are given.

(Lens Data) r₁ = ∞ d₁ = 0.6500 ne₁ = 1.51872 υe₁ = 64.00 r₂ = ∞ d₂ =0.5000 r₃ = ∞ (aperture stop) d₃ = 3.8293 r₄ = −9.4944 d₄ = 2.8640 ne₂ =1.83945 υe₂ = 42.47 r₅ = −5.8201 d₅ = 0.6500 ne₃ = 1.81184 υe₃ = 33.03r₆ = −16.1384 d₆ = 0.2000 r₇ = 22.3959 d₇ = 3.2615 ne₄ = 1.85639 υe₄ =39.85 (aspheric surface) r₈ = −18.3976 d₈ = D(8) (aspheric surface)(variable) r₉ = 19.3324 d₉ = 0.6500 ne₅ = 1.82917 υe₅ = 23.86 (asphericsurface) r₁₀ = 10.5762 d₁₀ = D(10) (aspheric surface) (variable) r₁₁ =−17.1418 d₁₁ = 4.1419 ne₆ = 1.74689 υe₆ = 49.07 (aspheric surface) r₁₂ =−12.0908 d₁₂ = 2.8770 (aspheric surface) r₁₃ = −22.5833 d₁₃ = 1.0000 ne₇= 1.83930 υe₇ = 37.09 r₁₄ = −77.3086 d₁₄ = Bf Constant of the Cone (k)and Aspheric Coefficients (A₄, A₆, A₈, A₁₀) (Seventh Order) k = 0, A₄ =−4.44632 × 10⁻⁵, A₆ = −3.60976 × 10⁻⁸, A₈ = 9.18001 × 10⁻⁹, A₁₀ =−2.80183 × 10⁻¹¹ (Eighth Order) k = 0, A₄ = 5.44540 × 10⁻⁵, A₆ = 2.72131× 10⁻⁷, A₈ = 3.22891 × 10⁻⁹, A₁₀ = 9.43255 × 10⁻¹² (Ninth Order) k = 0,A₄ = −2.70854 × 10⁻⁵, A₆ = −2.61994 × 10⁻⁶, A₈ = 2.06465 × 10⁻⁸, A₁₀ =−1.04244 × 10⁻¹⁰ (Tenth Order) k = 0, A₄ = 3.25644 × 10⁻⁵, A₆ = −3.06186× 10⁻⁶, A₈ = −3.22202 × 10⁻⁹, A₁₀ = 7.42390 × 10⁻¹¹ (Eleventh Order) k =0, A₄ = 1.76311 × 10⁻⁴, A₆ = 1.34885 × 10⁻⁶, A₈ = −1.04265 × 10⁻⁸, A₁₀ =3.35661 × 10⁻¹² (Twelfth Order) k = 0, A₄ = 1.19215 × 10⁻⁴, A₆ = 5.79903× 10⁻⁷, A₈ = 2.49765 × 10⁻⁹, A₁₀ = −1.46183 × 10⁻¹¹ (Focal State Data)Minimum Object Distance Infinity (object distance) D(8) 1.0688 1.8929D(10) 6.8075 5.9834 f (focal length of 27.5462 26.0246 optical systemoverall) F no. (F number) 2.8840 2.8985 ω (half angle of view) 38.588138.3233 Y (image height) 20.29 21.07 Bf (back focus) 15.6482 15.6482(Values Related to Conditional Expression (1)) f3 (focal length of thirdlens group G₁₃ at focus state for object at infinity) = −400.0000 f3/f =−14.52 (Values Related to Conditional Expression (2)) f1 (focal lengthof first lens group G₁₁ at focus state for object at infinity) = 13.6190f1/f = 0.49 (Values Related to Conditional Expression (3)) βinf(paraxial magnification of second lens group G₁₂ at focus state forobject at infinity) = 1.87 βmod (paraxial magnification of second lensgroup G₁₂ at focus state for minimum object distance) = 1.85 βinf/βmod =1.01 (Values Related to Conditional Expression (4)) R1 (radius ofcurvature of surface at air interface on object side of negative lensL₁₃₂) = −22.5833 R2 (radius of curvature of surface at air interface onimage side of negative lens L₁₃₂) = −77.3086 (R1 + R2)/(R1 − R2) = −1.83(Values Related to Conditional Expression (5)) L1s (axial distance fromsurface farthest on object side of first lens group G₁₁ to aperture stopS) = 1.1500 L (overall length of optical system) = 44.1482 L1s/L = 0.03(Values Related to Conditional Expression (6)) f2 (focal length ofsecond lens group G₁₂ at focus state for object at infinity) = −29.1423f2/f = −1.06 (Values Related to Conditional Expression (7)) When movedlens group (stabilizing group) is positive lens L₁₁₄ of first lens groupG₁₁ βp (transverse magnification of lens group moved in directionorthogonal to optical axis) = −0.33 βr (composite transversemagnification of lenses disposed farther on image side than lens groupmoved in direction orthogonal to optical axis) = 2.02 (1 − βp) × βr =2.69 When moved lens group (stabilizing group) is positive lens L₁₃₁ ofthird lens group G₁₃ βp (transverse magnification of lens group moved indirection orthogonal to optical axis) = 0.76 βr (composite transversemagnification of lenses disposed farther on image side than lens groupmoved in direction orthogonal to optical axis) = 1.43 (1 − βp) × βr =0.35 When moved lens group (stabilizing group) is all lenses of firstlens group G₁₁ , excluding lens L₁₁₁ having no refractive power βp(transverse magnification of lens group moved in direction orthogonal tooptical axis) = 0 βr (composite transverse magnification of lensesdisposed farther on image side than lens group moved in directionorthogonal to optical axis) = 1 (1 − βp) × βr = 1 (Values Related toConditional Expression (8)) υen (Abbe number for e-line of negative lensL₁₃₂) = 37.09 (Values Related to Conditional Expression (9)) R21 (radiusof curvature of surface farthest on object side of negative lens L₁₂₁) =19.3324 R22 (radius of curvature of surface farthest on image side ofnegative lens L₁₂₁) = 10.5762 (R21 + R22)/(R21 − R22) = 3.42

FIG. 2 is a diagram of various types of aberration occurring in theinner focus lens according to the first embodiment. In the diagram,curves depict wavelength aberration corresponding to the e-line(λ=546.074 nm). S and M shown with respect to astigmatism, respectivelyindicate aberration at the sagittal imaging plane and at the meridonalimaging plane.

FIG. 3 is a diagram depicting, along the optical axis, a configurationof the inner focus lens according to a second embodiment. FIG. 3 depictsa focus state for an object at infinity. The inner focus lens includessequentially from the object side, a first lens group G₂₁ having apositive refractive power, a second lens group G₂₂ having a negativerefractive power, and a third lens group G₂₃ having a negativerefractive power.

The first lens group G₂₁ includes sequentially from the object side, apositive lens L₂₁₁, the aperture stop S determining a given diameter, apositive lens L₂₁₂, a negative lens L₂₁₃, and a positive lens L₂₁₄. Thepositive lens L₂₁₂ and the negative lens L₂₁₃ are cemented. Bothsurfaces of the positive lens L₂₁₄ are aspheric.

The second lens group G₂₂ is configured by a negative lens L₂₂₁. Bothsurfaces of the negative lens L₂₂₁ are aspheric. The second lens groupG₂₂ is moved along the optical axis from the object side to the imageside, whereby focusing from a focus state for an object at infinity to afocus state for the minimum object distance is performed.

The third lens group G₂₃ includes sequentially from the object side, apositive lens L₂₃₁ (front sub-lens group) and a negative lens L₂₃₂ (rearsub-lens group). Both surfaces of the positive lens L₂₃₁ are aspheric.An air gap is formed between the positive lens L₂₃₁ and the negativelens L₂₃₂.

In the inner focus lens according to the second embodiment, the positivelens L₂₁₄ included in the first lens group G₂₁ or the positive lens L₂₃₁included in the third lens group G₂₃ is moved in a direction orthogonalto the optical axis, whereby stabilization correction is performed.

Here, various values related to the inner focus lens according to thesecond embodiment are given.

(Lens Data) r₁ = 17.9780 d₁ = 3.1723 ne₁ = 1.49845 υe₁ = 81.21 r₂ =−179.8468 d₂ = 0.5000 r₃ = ∞ (aperture stop) d₃ = 3.7714 r₄ = −24.8117d₄ = 1.6114 ne₂ = 1.49845 υe₂ = 81.21 r₅ = −17.9439 d₅ = 0.6500 ne₃ =1.73432 υe₃ = 28.10 r₆ = 47.4504 d₆ = 0.6736 r₇ = 28.5683 d₇ = 2.9298ne₄ = 1.88765 υe₄ = 36.97 (aspheric surface) r₈ = −23.6412 d₈ = D(8)(aspheric surface) (variable) r₉ = 38.5730 d₉ = 0.6500 ne₅ = 1.62518 υe₅= 57.96 (aspheric surface) r₁₀ = 12.3652 d₁₀ = D(10) (aspheric surface)(variable) r₁₁ = −19.3415 d₁₁ = 1.4124 ne₆ = 2.00912 υe₆ = 28.91(aspheric surface) r₁₂ = −17.3396 d₁₂ = 8.4378 (aspheric surface) r₁₃ =−13.9954 d₁₃ = 1.0000 ne₇ = 1.58481 υe₇ = 40.61 r₁₄ = −20.2884 d₁₄ = BfConstant of the Cone (k) and Aspheric Coefficients (A₄, A₆, A₈, A₁₀)(Seventh Order) k = 0, A₄ = −5.31140 × 10⁻⁵, A₆ = 1.77167 × 10⁻⁸, A₈ =−2.98858 × 10⁻¹⁰, A₁₀ = 2.27493 × 10⁻¹¹ (Eighth Order) k = 0, A₄ =3.94940 × 10⁻⁶, A₆ = −5.26041 × 10⁻⁸, A₈ = −8.87342 × 10⁻¹¹, A₁₀ =1.87394 × 10⁻¹¹ (Ninth Order) k = 0, A₄ = 7.42096 × 10⁻⁶, A₆ = −1.53328× 10⁻⁶, A₈ = 1.49734 × 10⁻⁸, A₁₀ = −4.71441 × 10⁻¹¹ (Tenth Order) k = 0,A₄ = 1.80928 × 10⁻⁵, A₆ = −1.60282 × 10⁻⁶, A₈ = 6.53719 × 10⁻⁹, A₁₀ =3.47436 × 10⁻¹² (Eleventh Order) k = 0, A₄ = 1.40765 × 10⁻⁴, A₆ =4.99455 × 10⁻⁷, A₈ = −1.94373 × 10⁻⁹, A₁₀ = −9.37987 × 10⁻¹² (TwelfthOrder) k = 0, A₄ = 1.04350 × 10⁻⁴, A₆ = 3.22665 × 10⁻⁷, A₈ = 6.06200 ×10⁻¹⁰, A₁₀ = −1.72843 × 10⁻¹¹ (Focal State Data) Minimum Object DistanceInfinity (object distance) D(8) 1.2021 3.4333 D(10) 6.9558 4.7246 f(focal length of 48.4962 40.8401 optical system overall) F no. (Fnumber) 2.8840 2.9589 ω (half angle of view) 21.9113 20.7990 Y (imageheight) 21.63 21.63 Bf (back focus) 16.1817 16.1817 (Values Related toConditional Expression (1)) f3 (focal length of third lens group G₂₃ atfocus state for object at infinity) = −263.2280 f3/f = −5.43 (ValuesRelated to Conditional Expression (2)) f1 (focal length of first lensgroup G₂₁ at focus state for object at infinity) = 22.0829 f1/f = 0.46(Values Related to Conditional Expression (3)) βinf (paraxialmagnification of second lens group G₂₂ at focus state for object atinfinity) = 2.09 βmod (paraxial magnification of second lens group G₂₂at focus state for minimum object distance) = 2.02 βinf/βmod = 1.04(Values Related to Conditional Expression (4)) R1 (radius of curvatureof surface at air interface on object side of negative lens L₂₃₂) =−13.9954 R2 (radius of curvature of surface at air interface on imageside of negative lens L₂₃₂) = −20.2884 (R1 + R2)/(R1 − R2) = −5.45(Values Related to Conditional Expression (5)) L1s (axial distance fromsurface farthest on object side of first lens group G₂₁ to aperture stopS) = 3.6723 L (overall length of optical system) = 49.1482 L1s/L = 0.07(Values Related to Conditional Expression (6)) f2 (focal length ofsecond lens group G₂₂ at focus state for object at infinity) = −29.3910f2/f = −0.61 (Values Related to Conditional Expression (7)) When movedlens group (stabilizing group) is positive lens L₂₁₄ of first lens groupG₂₁ βp (transverse magnification of lens group moved in directionorthogonal to optical axis) = −0.19 βr (composite transversemagnification of lenses disposed farther on image side than lens groupmoved in direction orthogonal to optical axis) = 2.20 (1 − βp) × βr =2.62 When moved lens group (stabilizing group) is positive lens L₂₃₁ ofthird lens group G₂₃ βp (transverse magnification of lens group moved indirection orthogonal to optical axis) = 0.86 βr (composite transversemagnification of lenses disposed farther on image side than lens groupmoved in direction orthogonal to optical axis) = 1.22 (1 − βp) × βr =0.17 (Values Related to Conditional Expression (8)) υen (Abbe number fore-line of negative lens L₂₃₂) = 40.61 (Values Related to ConditionalExpression (9)) R21 (radius of curvature of surface farthest on objectside of negative lens L₂₂₁) = 38.5730 R22 (radius of curvature ofsurface farthest on image side of negative lens L₂₂₁) = 12.3652 (R21 +R22)/(R21 − R22) = 1.94

FIG. 4 is a diagram of various types of aberration occurring in theinner focus lens according to the second embodiment. In the diagram,curves depict wavelength aberration corresponding to the e-line(λ=546.074 nm). S and M shown with respect to astigmatism, respectivelyindicate aberration at the sagittal imaging plane and at the meridonalimaging plane.

FIG. 5 is a diagram depicting, along the optical axis, a configurationof the inner focus lens according to a third embodiment. FIG. 5 depictsa focus state for an object at infinity. The inner focus lens includessequentially from the object side, a first lens group G₃₁ having apositive refractive power, a second lens group G₃₂ having a negativerefractive power, and a third lens group G₃₃ having a negativerefractive power.

The first lens group G₃₁ includes sequentially from the object side, apositive lens L₃₁₁, the aperture stop S determining a given diameter, anegative lens L₃₁₂, and a positive lens L₃₁₃. Both surfaces of thepositive lens L₃₁₃ are aspheric.

The second lens group G₃₂ is configured by a negative lens L₃₂₁. Bothsurfaces of the negative lens L₃₂₁ are aspheric. The second lens groupG₃₂ is moved along the optical axis from the object side to the imageside, whereby focusing from a focus state for an object at infinity to afocus state for the minimum object distance is performed.

The third lens group G₃₃ includes sequentially from the object side, apositive lens L₃₃₁ (front sub-lens group) and a negative lens L₃₃₂ (rearsub-lens group). Both surfaces of the positive lens L₃₃₁ are aspheric.An air gap is formed between the positive lens L₃₃₁ and the negativelens L₃₃₂.

In the inner focus lens according to the third embodiment, the positivelens L₃₁₃ included in the first lens group G₃₁ or the positive lens L₃₃₁included in the third lens group G₃₃ is moved in a direction orthogonalto the optical axis, whereby stabilization correction is performed.

Here, various values related to the inner focus lens according to thethird embodiment are given.

(Lens Data) r₁ = 16.4363 d₁ = 5.5000 ne₁ = 1.49845 υe₁ = 81.21 r₂ =−300.0000 d₂ = 0.5000 r₃ = ∞ d₃ = 1.8212 (aperture stop) r₄ = −44.7468d₄ = 0.7000 ne₂ = 1.72310 υe₂ = 29.27 r₅ = 373.5699 d₅ = 0.8417 r₆ =20.8910 d₆ = 3.3714 ne₃ = 1.49856 υe₃ = 81.16 (aspheric surface) r₇ =−33.7365 d₇ = D(7) (aspheric surface) (variable) r₈ = 90.3752 d₈ =0.6500 ne₄ = 1.58547 υe₄ = 59.22 (aspheric surface) r₉ = 11.0161 d₉ =D(9) (aspheric surface) (variable) r₁₀ = −23.8098 d₁₀ = 1.3706 ne₅ =1.82917 υe₅ = 23.86 (aspheric surface) r₁₁ = −18.3476 d₁₁ = 7.8919(aspheric surface) r₁₂ = −13.3351 d₁₂ = 0.7000 ne₆ = 1.49845 υe₆ = 81.21r₁₃ = −22.5577 d₁₃ = Bf Constant of the Cone (k) and AsphericCoefficients (A₄, A₆, A₈, A₁₀) (Sixth Order) k = 0, A₄ = −6.66384 ×10⁻⁵, A₆ = −3.55330 × 10⁻⁷, A₈ = 6.24915 × 10⁻¹⁰, A₁₀ = −8.69024 × 10⁻¹²(Seventh Order) k = 0, A₄ = 4.36278 × 10⁻⁶, A₆ = −1.41704 × 10⁻⁷, A₈ =−1.53397 × 10⁻¹⁰, A₁₀ = −3.28298 × 10⁻¹² (Eighth Order) k = 0, A₄ =−1.17365 × 10⁻⁵, A₆ = −9.52960 × 10⁻⁸, A₈ = 5.84886 × 10⁻¹⁰, A₁₀ =−3.14094 × 10⁻¹² (Ninth Order) k = 0, A₄ = −9.90628 × 10⁻⁶, A₆ =−6.68574 × 10⁻⁷, A₈ = 8.71141 × 10⁻⁹, A₁₀ = −6.97618 × 10⁻¹¹ (TenthOrder) k = 0, A₄ = 8.76623 × 10⁻⁵, A₆ = −4.76558 × 10⁻⁷, A₈ = 1.66193 ×10⁻⁸, A₁₀ = −8.96128 × 10⁻¹¹ (Eleventh Order) k = 0, A₄ = 5.77887 ×10⁻⁵, A₆ = −5.30380 × 10⁻⁷, A₈ = 1.24226 × 10⁻⁸, A₁₀ = −7.91033 × 10⁻¹¹(Focal State Data) Minimum Object Distance Infinity (object distance)D(7) 1.2117 3.0915 D(9) 6.4767 4.5969 f (focal length of 58.5031 46.1351optical system overall) F no. (F number) 2.8840 3.0166 ω (half angle ofview) 18.4621 17.3034 Y (image height) 21.63 21.63 Bf (back focus)20.1130 20.1130 (Values Related to Conditional Expression (1)) f3 (focallength of third lens group G₃₃ at focus state for object at infinity) =−400.0000 f3/f = −6.84 (Values Related to Conditional Expression (2)) f1(focal length of first lens group G₃₁ at focus state for object atinfinity) = 21.4980 f1/f = 0.37 (Values Related to ConditionalExpression (3)) βinf (paraxial magnification of second lens group G₃₂ atfocus state for object at infinity) = 2.73 βmod (paraxial magnificationof second lens group G₃₂ at focus state for minimum object distance) =2.65 βinf/βmod = 1.03 (Values Related to Conditional Expression (4)) R1(radius of curvature of surface at air interface on object side ofnegative lens L₃₃₂) = −13.3351 R2 (radius of curvature of surface at airinterface on image side of negative lens L₃₃₂) = −22.5577 (R1 + R2)/(R1− R2) = −3.89 (Values Related to Conditional Expression (5)) L1s (axialdistance from surface farthest on object side of first lens group G₃₁toaperture stop S) = 6.0000 L (overall length of optical system) = 51.1482L1s/L = 0.12 (Values Related to Conditional Expression (6)) f2 (focallength of second lens group G₃₂ at focus state for object at infinity) =−21.4928 f2/f = −0.37 (Values Related to Conditional Expression (7))When moved lens group (stabilizing group) is positive lens L₃₁₃ of firstlens group G₃₁ βp (transverse magnification of lens group moved indirection orthogonal to optical axis) = 0.37 βr (composite transversemagnification of lenses disposed farther on image side than lens groupmoved in direction orthogonal to optical axis) = 2.72 (1 − βp) × βr =1.72 When moved lens group (stabilizing group) is positive lens L₃₃₁ ofthird lens group G₃₃ βp (transverse magnification of lens group moved indirection orthogonal to optical axis) = 0.76 βr (composite transversemagnification of lenses disposed farther on image side than lens groupmoved in direction orthogonal to optical axis) = 1.32 (1 − βp) × βr =0.32 (Values Related to Conditional Expression (8)) υen (Abbe number fore-line of negative lens L₃₃₂) = 81.21 (Values Related to ConditionalExpression (9)) R21 (radius of curvature of surface farthest on objectside of negative lens L₃₂₁) = 90.3752 R22 (radius of curvature ofsurface farthest on image side of negative lens L₃₂₁) = 11.0161 (R21 +R22)/(R21 − R22) = 1.28

FIG. 6 is a diagram of various types of aberration occurring in theinner focus lens according to the third embodiment. In the diagram,curves depict wavelength aberration corresponding to the e-line(λ=546.074 nm). S and M shown with respect to astigmatism, respectivelyindicate aberration at the sagittal imaging plane and at the meridonalimaging plane.

Among the values for each of the embodiments, r₁, r_(2r), . . . indicatethe radius of curvature of lens surfaces, diaphragm surface, etc.; d₁,d₂, . . . indicate the thickness of the lenses, the diaphragm, etc. orthe interval between the surfaces thereof; ne₁, ne₂, . . . indicate therefraction index of the lenses with respect to the e-line (λ=546.074nm); and νe₁, νe₂, . . . indicate the Abbe number for the e-line(λ=587.56 nm) of the lenses. Lengths are indicated in units of “mm”; andangles are indicated in “degrees”.

Each aspheric surface shape above is expressed by the equation below;where, Z is the depth of the aspheric surface, c(1/r) is curvature; h isthe height from the optical axis; k is the constant of the cone; A₄, A₆,A₈, A₁₀ are respectively fourth order, sixth order, eighth order, andtenth order aspheric coefficients; and the travel direction of light isassumed to be positive.

$Z = {\frac{{ch}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}h^{2}}}} + {A_{4}h^{4}} + {A_{6}h^{6}} + {A_{8}h^{8}} + {A_{10}h^{10}}}$

In the embodiments, an example of an inner focus lens having focallengths accommodating wide angles to standard angles of view by a 35 mmfilm camera conversion has been given. The inner focus lens of theembodiments facilitates reductions in the size and weight of thefocusing group and therefore, can favorably perform high-speed autofocusprocessing, essential for video filming. Further, since the distancethat the stabilizing group is moved during stabilization correction canbe suppressed, increases in the optical system diameter can besuppressed. In particular, by satisfying the conditional expressionsabove, an inner focus lens that is optimal for video filming, iscompact, and has high imaging performance at wide angles can berealized.

The inner focus lens according to the present embodiment is useful forcompact imaging apparatuses such as still cameras, video cameras, etc.and in particular, is optimal for imaging apparatuses for video filming.

According to the embodiments, a high imaging performance inner focuslens for which reduction of the overall length and diameter is achievedcan be provided.

According to the embodiments, an inner focus lens that has focal lengthsaccommodating wide angles to standard angles of view and high imagingperformance can be provided.

According to the embodiments, variations in the angle of view consequentto focusing are suppressed, enabling improved imaging performance.

According to the embodiments, the diameter of a lens near the imagingplane is reduced and axial as well as off axis aberration (particularly,distortion) can be favorably corrected.

According to the embodiments, the diameter of the third lens group (lensfarthest on the image side) is reduced and off axis coma can befavorably corrected.

According to the embodiments, the diameters of the front lens and therear lens are reduced while maintaining imaging performance, enablingsize reductions of the optical system to be facilitated.

According to the embodiments, reductions in the size and weight of thesecond lens group, which is the focusing group, enables an inner focuslens to be provided that is applicable to video filming.

According to the embodiments, the overall length of the optical systemcan be reduced and imaging performance can be improved.

According to the embodiments, a compact inner focus lens can be providedthat has a stabilization correction function. In particular, thedistance that the stabilizing group is moved during stabilizationcorrection can be suppressed, the diameter of the optical system can bereduced, and stabilization correction performance can be improved.

According to the embodiments, reductions in the size and weight of thesimple lens element having a negative refractive power and disposedfarthest on the image side of the third lens group become easy andchromatic difference of magnification can be favorably corrected.

According to the present invention, an inner focus lens having highimaging performance and for which reduction of the overall length anddiameter is achieved can be provided. Furthermore, an inner focus lensthat has focal lengths accommodating wide angles to standard angles ofview and high imaging performance can be provided. According to thepresent invention, a compact inner focus lens that is optimal for videofilming can be provided.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

The present document incorporates by reference the entire contents ofJapanese priority document, 2014-039818 filed in Japan on Feb. 28, 2014.

What is claimed is:
 1. An inner focus lens comprising sequentially froman object side: a first lens group having a positive refractive power; asecond lens group having a negative refractive power; and a third lensgroup having a negative refractive power, wherein the second lens groupis moved along an optical axis, whereby focusing from a focus state foran object at infinity to a focus state for a minimum object distance isperformed, and the inner focus lens satisfies a conditional expression(1) −29.0≦f3/f≦−5.4, where f3 is a focal length of the third lens groupat the focus state for an object at infinity and f is a focal length ofthe optical system overall at the focus state for an object at infinity.2. The inner focus lens according to claim 1, wherein the inner focuslens satisfies a conditional expression (2) 0.18≦f1/f≦0.99, where f1 isa focal length of the first lens group at the focus state for an objectat infinity.
 3. The inner focus lens according to claim 1, wherein theinner focus lens satisfies a conditional expression (3)0.51≦βinf/βmod≦2.07, where βinf is paraxial magnification of the secondlens group at the focus state for an object at infinity and βmod isparaxial magnification of the second lens group at the focus state forthe minimum object distance.
 4. The inner focus lens according to claim1, wherein the third lens group includes sequentially from the objectside: a front sub-lens group having a positive refractive power, and arear sub-lens group having a negative refractive power; and an axial airgap that is widest in the third lens group is formed between the frontsub-lens group and the rear sub-lens group.
 5. The inner focus lensaccording to claim 1, wherein the third lens group includes a simplelens element having a negative refractive power and disposed farthest onan image side, and the inner focus lens satisfies a conditionalexpression (4) (R1+R2)/(R1−R2)≦0.0, where R1 is radius of curvature of asurface at an air interface on the object side of the simple lenselement having a negative refractive power and R2 is radius of curvatureof a surface at an air interface on the image side of the simple lenselement having a negative refractive power.
 6. The inner focus lensaccording to claim 1, wherein the inner focus lens satisfies aconditional expression (5) 0.01≦L1s/L≦0.53, where L1s is an axialdistance from a surface farthest on the object side of the first lensgroup to an aperture stop and L is overall optical system length (anair-conversion optical path length from an apex of a lens surfacefarthest on the object side to an imaging plane).
 7. The inner focuslens according to claim 1, wherein the second lens group is configuredby a simple lens element having a negative refractive power.
 8. Theinner focus lens according to claim 1, wherein the inner focus lenssatisfies a conditional expression (6) −2.12≦f2/f≦−0.18, where f2 is afocal length of the second lens group at the focus state for an objectat infinity.
 9. The inner focus lens according to claim 1, wherein alens group configured by a lens other than a lens that is disposedfarthest on the object side is moved in a direction orthogonal to theoptical axis to shift an image, and the inner focus lens satisfies aconditional expression (7) 0.15≦(1−βp)×βr≦4.50, where βp is transversemagnification of the lens group moved in a direction orthogonal to theoptical axis and βr is composite transverse magnification of a lensgroup disposed farther on an image side than the lens group moved in adirection orthogonal to the optical axis.
 10. The inner focus lensaccording to claim 7, wherein the simple lens element having a negativerefractive power and disposed farthest on the image side of the thirdlens group is configured by a simple glass material, and the inner focuslens satisfies a conditional expression (8) 30≦νen, where νen is an Abbenumber for e-line of the simple lens element having a negativerefractive power and disposed farthest on the image side of the thirdlens group.